The hot embossing foil printing process herein referred to is a dry printing method in which the hot embossing foil adheres to or, respctively, is melted onto the surface to be printed. The hot embossing foil itself consists of a carrier strip, a separating layer, advisably a protective film, the actual ink film, which often contains an additional metallic coating, and the adhesive layer or layer for connecting to the surface which is to be printed, which usually consists of plastic material.
Up until now in hot embossing technology, two main methods have been employed, i.e. on the one hand, the lifting process and, on the other hand, the unrolling process. In the lifting process the workpiece is held in place and the embossing die moved to and fro in a strokelike (lifting) fashion. The embossing die represents a rigid body. In the unrolling process, which is especially used for surface coating of cylindrical or slightly conical parts such as lipstick tubes, jars for cream or similar, the workpiece is moved and unrolled on the embossing die with a line contact. The length of the embossing die corresponds to the embossing development. In doing this a substantial contact pressure must also be attained so that the necessary temperature is achieved within the short time available for the unrolling process.
From DE-PS 34 21 029 a combined lifting/unrolling process is known in which the embossing die is brought into contact with the workpiece through continuous engagement over the entire die surface and, in doing this, the embossing die is given a form corresponding to the shape of the workpiece at least in the region of the die surface. Here, the direction of movement of the continuous engagement is directed perpendicular to the working direction, i.e. to the direction in which the individual workpieces to be embossed are guided through the corresponding apparatus. Thus, workpiece and embossing die are moved relatively towards each other, whereby the workpiece often remains absolutely stationary. However, the embossing die, by virtue of its shape, its characteristic thin-walls and through use of flexible material for its production, is already so yielding that this flexibility can be exploited here in order to, as it were, apply and shape the embossing die to the workpiece during the embossing process. During this procedure this engagement process brings evermore larger surface regions into contact with each other so that, advantageously, the compensation for unevenness of the workpiece and a corresponding by-passing of tolerances is possible. Also with this known method, the position of the printed image can be easily altered and differing hollow body shapes can be served with the same embossing die. Printing position modifications are not a problem. However, a disadvantage with this known method is that different surface regions come into contact with the workpiece successively resulting in differing contact times in the region of the die surface of the embossing die. In particular, with short contact times, like those which are essential for a correspondingly high embossing performance, relatively large differences in the contact times associated with the individual surface regions are the result. These differences in contact times are a disadvantage in every respect because they have a negative surface-zonal influence on the printed image. For example, the greater the angle of contact with a bottle which is to be embossed, the more seriously noticable are these differences in contact times. With the known method it is not possible to provide, for example, a hollow body, with hot embossed printing over the entire perimeter, i.e. over 360.degree..
An apparatus for dry printing of a workpiece using a hot embossing foil is known from DE-PS 38 29 297 in which the die body of the embossing die and, ultimately, the complete embossing die are constructed so thin and flexible that a local elastic deformation of the embossing die is possible upon engagement with the workpiece in order to equalize the prominent parts and recesses, and therewith to minimize the waste. The embossing die is fitted with a positive force transfer relief on the rear side of the die surface and a pressure pad is used for the local elastic shaping. This aims to keep the thermal and mechanical load on the sensitive embossing die as low as possible. It is thereby possible, advantageously, to bring about a well-directed force distribution for the contact pressure during the embossing process, and to compensate for recesses, unevenness and/or differences in wall thickness, in particular with yielding hollow bodies. It is possible, through a well-directed partial heating of the embossing die, to locally influence the heat transfer. The die surface can also be situated on an endless belt, whereby the embossing process is then carried out with line contact. However, with unrolling, such a line contact demands relatively high temperatures for the embossing die owing to the necessarily short contact time. However, particularly high temperatures damage the embossing die and lead to a reduction in the service life. The arrangement of a positive force transfer relief on the rear side of the die surface produces a certain complication and increase in the production expenses for the embossing die.